Wiper Die Clearance and Angle for Inside Radius Wrinkles

Wrinkles on the inside radius usually appear when the tube wall can no longer stay controlled as it enters the bend die. In rotary draw bending, unsupported material in the compressive zone yields under axial strain, forming wave-like instability along the intrados. While internal support tools stabilize the inner circumference, controlling the gap between the wiper die tip and the bend die tangent point is necessary for thin-wall or tight-radius applications.

Dural Machinery has manufactured tube processing equipment in Istanbul, Gungoren since 1991. In field service, incorrect wiper die orientation is responsible for a high proportion of recurring surface defects. Eliminating inside-radius wrinkles requires setting the geometric rake angle, controlling mechanical clearance, and maintaining rigid backing alignment.

Wrinkle formation on the inside bend radius

During the bending cycle, the outer wall of the tube stretches under tension while the inner wall compresses under axial thrust. When the tube wall ratio (outer diameter divided by wall thickness) increases or the center line radius decreases, compressive forces exceed the structural stability of the unsupported tube segment. Without external support, the sheet metal buckles upward, creating periodic ridges along the inside bend.

The pressure die holds the trailing section of the tube, while the clamp die secures the leading end to the rotating bend die. However, between the pressure die tip and the point where the tube contacts the bend die cavity, an open triangle of unsupported metal exists. If the material enters this region without containment, compressive stress pushes the tube wall outward into the open space before it fully seats into the bend die groove.

The function of the wiper die is to close this void. Mounted directly behind the bend die, its thin feather edge extends into the tangent zone to maintain continuous contact with the tube intrados. If the clearance between this edge and the bend die is excessive, the tube material forces its way under the tip, forming localized humps or full longitudinal wrinkles.

Setting the rake angle and tip clearance

Positioning a wiper die involves balancing two physical boundaries: providing enough contact force to suppress wall buckling without causing excessive drag, scraping, or tool tip deflection. The configuration requires precise setting of both the rake angle and the tip distance relative to the bend die tangent point.

The rake angle is the small tilt applied to the wiper die holder relative to the straight tube axis. In most setups, this angle is set between 0.5 degrees and 1.5 degrees. Tilting the wiper die slightly away from the tube behind the tip reduces friction along the rear body of the tool while keeping the leading tip pressed firmly against the bend die. If the rake angle is set parallel (0 degrees), friction increases along the entire length of the wiper die, causing galling, machine overload, or tool failure.

Tip clearance defines how far behind the theoretical tangent point the feather edge sits. For standard steel and stainless steel bending, the tip is positioned approximately 0.5 mm to 1.5 mm back from the true tangent point. Moving the tip too far forward increases the risk of pinching the wiper edge against the bend die groove, which destroys the delicate feather edge. Setting the tip too far back leaves an uncontained gap where compression wrinkles initiate.

On automated equipment such as the CNC Boru Bükme Makinesi (Griffin), which features 3 to 6 axes and the ETU control interface across 11 models handling Ø32 to Ø170 mm tubing, structural rigidity ensures that the programmed alignment remains stable under continuous load. Hydraulic clamp systems must maintain consistent holding force to prevent micro-slippage that would otherwise alter the true position relative to the wiper edge.

Measuring wear and pocket deformation

Wiper dies are subject to localized wear, particularly along the delicate feather edge. Over prolonged production runs, friction from raw tubing wears down the sharp tip radius, effectively increasing the clearance gap even if the tool holder position remains fixed.

This is one of the first things we check when a customer reports wrinkles at the start of the bend. Inspecting the tool under magnification often reveals that the crisp zero-radius edge has eroded back into a rounded lip. Once the tip rounds over, compressive forces force tube material into the newly created clearance zone, forming fine ripples that worsen as tool temperature rises.

Tool material selection affects how rapidly this edge degradation occurs:

• Steel wiper dies: Suitable for non-ferrous tubes like copper or aluminum. When used on stainless steel, lubrication failure can cause galling.
• Bronze alloy wiper dies: Preferred for stainless steel, titanium, and high-strength alloy tubing. The softer bronze acts as a sacrificial surface, preventing material transfer to the workpiece.
• Insert-type wiper dies: Feature replaceable alloy tips mounted to a steel body, reducing tool replacement costs on high-volume production lines.

Regular maintenance includes checking the wiper pocket on the bend die. If the bend die groove has developed side-wall deflection or uneven wear, the wiper die cannot seat flush against the radius. A feeler gauge should be used across the interface to verify uniform contact across the entire arc before starting a production run.

Field verification on CNC tube benders

Achieving defect-free bends requires verifying mechanical setup prior to running automatic cycles. Mechanical alignment on machines such as the Yarı Otomatik Boru Bükme Makinesi (Atlas BR) or fully electric models relies on methodical mechanical checks combined with precise axis setup.

Follow this checklist when setting up wiper tooling:

1. Mount the bend die and clamp die, ensuring all mounting surfaces are free of debris and torqued to specification.
2. Insert the wiper die into its mounting bracket without fully tightening the locking bolts.
3. Advance the wiper die forward until the feather edge lightly touches the bend die cavity at the tangent point.
4. Adjust the rake adjustment screw to tilt the rear of the wiper die away from the tube line by 0.5 to 1.0 degree.
5. Retract the tip slightly using a feeler gauge to set a uniform clearance gap of 0.5 mm behind tangent.
6. Tighten all backing screws securely to prevent tool push-back under bending force.
7. Apply high-pressure bending lubricant to both the internal mandrel and the wiper die contact zone.

If wrinkles persist after setting correct geometry, check that the pressure die assist speed matches tube advance and verify that the mandrel nose is not positioned too far behind tangent. Thin-wall bending requires synchronized interaction between all four primary tooling components: clamp die, pressure die, mandrel, and wiper die.

To evaluate tooling recommendations for specific tube geometries, submit your scaled drawing or STEP file along with material grade, wall thickness, center line radius, and target annual production volume.